make sure we always save database as KDBX4 when using ChallengeXCKey (for compatibility with KeepassXC), closes #596

src/KeePassLib2Android/Keys/IUserKey.cs

@@ -42,5 +42,7 @@ namespace KeePassLib.Keys
         // /// Clear the key and securely erase all security-critical information.
         // /// </summary>
         // void Clear();
+
+        uint GetMinKdbxVersion();
 	}
 }

src/KeePassLib2Android/Keys/KcpCustomKey.cs

@@ -45,6 +45,11 @@ namespace KeePassLib.Keys
 			get { return m_pbKey; }
 		}
 
+	    public uint GetMinKdbxVersion()
+	    {
+	        return 0;
+	    }
+
 	    public KcpCustomKey(string strName, byte[] pbKeyData, bool bPerformHash)
 		{
 			Debug.Assert(strName != null); if(strName == null) throw new ArgumentNullException("strName");

src/KeePassLib2Android/Keys/KcpKeyFile.cs

@@ -64,6 +64,11 @@ namespace KeePassLib.Keys
 			get { return m_pbKeyData; }
 		}
 
+	    public uint GetMinKdbxVersion()
+	    {
+	        return 0;
+	    }
+
 	    public IOConnectionInfo Ioc
 		{
 			get { return m_ioc; }

src/KeePassLib2Android/Keys/KcpPassword.cs

@@ -53,6 +53,11 @@ namespace KeePassLib.Keys
 			get { return m_pbKeyData; }
 		}
 
+	    public uint GetMinKdbxVersion()
+	    {
+	        return 0;
+	    }
+
 	    public KcpPassword(byte[] pbPasswordUtf8)
 		{
 			SetKey(pbPasswordUtf8);

src/KeePassLib2Android/Keys/KcpUserAccount.cs

@@ -60,6 +60,11 @@ namespace KeePassLib.Keys
 			get { return m_pbKeyData; }
 		}
 
+	    public uint GetMinKdbxVersion()
+	    {
+	        return 0;
+	    }
+
 	    /// <summary>
 		/// Construct a user account key.
 		/// </summary>

src/KeePassLib2Android/Serialization/KdbxFile.cs

@@ -23,6 +23,7 @@ using System.Diagnostics;
 using System.Drawing;
 using System.Globalization;
 using System.IO;
+using System.Linq;
 using System.Security;
 using System.Text;
 using System.Xml;
@@ -126,8 +127,8 @@ namespace KeePassLib.Serialization
 		/// </summary>
 		private const uint FileVersion32 = 0x00040000;
 
-		internal const uint FileVersion32_4 = 0x00040000; // First of 4.x series
-		internal const uint FileVersion32_3 = 0x00030001; // Old format 3.1
+		public const uint FileVersion32_4 = 0x00040000; // First of 4.x series
+		public const uint FileVersion32_3 = 0x00030001; // Old format 3.1
 
 		private const uint FileVersionCriticalMask = 0xFFFF0000;
 
@@ -373,13 +374,16 @@ namespace KeePassLib.Serialization
 			if(m_uForceVersion != 0) return m_uForceVersion;
 
             // See also KeePassKdb2x3.Export (KDBX 3.1 export module)
+		    uint minVersionForKeys = m_pwDatabase.MasterKey.UserKeys.Select(key => key.GetMinKdbxVersion()).Max();
 		    
             AesKdf kdfAes = new AesKdf();
 			if(!kdfAes.Uuid.Equals(m_pwDatabase.KdfParameters.KdfUuid))
-				return FileVersion32;
+				return Math.Max(FileVersion32, minVersionForKeys);
 
 			if(m_pwDatabase.PublicCustomData.Count > 0)
-				return FileVersion32;
+			    return Math.Max(FileVersion32, minVersionForKeys);
+
+
 
             bool bCustomData = false;
 			GroupHandler gh = delegate(PwGroup pg)
@@ -396,9 +400,10 @@ namespace KeePassLib.Serialization
 			};
 			gh(m_pwDatabase.RootGroup);
 			m_pwDatabase.RootGroup.TraverseTree(TraversalMethod.PreOrder, gh, eh);
-			if(bCustomData) return FileVersion32;
+			if(bCustomData)
+			    return Math.Max(FileVersion32, minVersionForKeys);
 
-			return FileVersion32_3; // KDBX 3.1 is sufficient
+            return Math.Max(FileVersion32_3, minVersionForKeys); ; // KDBX 3.1 is sufficient
 		}
 
 		private void ComputeKeys(out byte[] pbCipherKey, int cbCipherKey,

src/keepass2android/ChallengeXCKey.cs

@@ -2,6 +2,7 @@ using Java.Lang;
 using KeePassLib.Cryptography;
 using KeePassLib.Keys;
 using KeePassLib.Security;
+using KeePassLib.Serialization;
 using Exception = System.Exception;
 
 namespace keepass2android
@@ -62,6 +63,11 @@ namespace keepass2android
             }
         }
 
+        public uint GetMinKdbxVersion()
+        {
+            return KdbxFile.FileVersion32_4;
+        }
+
         private byte[] _kdfSeed;
 
         public ChallengeXCKey(LockingActivity activity, int requestCode)